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Prevention of Plant Disease

CONTROL MEASURES

The methods used to control plant diseases are many and varied, but the best protection against such troubles is to be conversant with the needs of the crop being grown, and to practise good cultivation so that the plants make healthy, vigorous growth. Robust plants-have some resistance to disease, but those weakened by adverse conditions are more likely to be attacked by parasitic fungi.

Many factors favour the spread of disease: buying cheap seed of poor quality from which weak seedlings are raised; growing plants too close together; acid soil; poor light in greenhouses; overfeeding with artificial manures; high temperatures. So far as fungus diseases are concerned, there is no substitute for good cultivation. Spraying, dusting, fumigating and seed treatments, important though they are, are only secondary.

Virus diseases present a rather different problem, for even well-grown plants can be severely affected.

GOOD CULTIVATION

The following are some of the factors of good cultivation, so important in the maintenance of clean, disease-free crops: Good soil drainage and correct soil preparation — if necessary, such humus-forming materials as peat and leaf mould can be added to improve the drainage; the supply of fertilizers, when necessary, so that plant foods are available when-ever needed; the correct ventilation of frames and greenhouses so that the excessively warm and humid conditions which encourage plant parasites are never experienced; careful pruning to provide more air circulation in the open garden, for stagnant air encourages such destructive diseases as apple scab and American gooseberry mildew.

CROP ROTATION

Crop rotation is important in keeping down disease. If the same kind of crop is grown in the same ground for several years in succession, two undesirable things happen. First, the same plant foods are being taken continually from the soil, and second, if the particular crop being grown has a disease, then the disease will be encouraged to build up in the soil.

Plant parasites live on small pieces of crop debris (for example, on fragments of stem, roots and so on) and most, to ensure their survival in winter, produce some special fruit body of a hard, resistant nature. Routine cultivation, therefore, spreads infected material and the ground becomes heavily infected.

But most diseases are confined to a particular crop and can be ‘starved out’ of the soil if different crops are grown in rotation. Under glass, where crop rotation may not be practicable, soil sterilization is essential.

WEEDS

Weeds act as alternate host plants to many fungus diseases which persist on them and then move on to cultivated plants. For instance, shepherd’s purse may carry the club root disease of cabbages, wallflowers and other cruciferous plants; and wild celery the serious leaf spot disease of cultivated celery. Greater plantain may carry the virus of spotted wilt, and white bryony may harbour the cucumber and marrow mosaic virus.

HYGIENE

Remove and burn all dead and decaying plant material, especially from frames and greenhouses. Remove and burn dead and dying branches from trees and shrubs and paint the wounds with a recognized protective preparation. Make sure that disease-carrying packing material and infected vegetable debris is not added to the compost heap.

PRUNING CUTS

Paint with protective material, such as white lead paint, all large pruning cuts of in. diameter or over. This is especially important if the cut is low down on the tree or shrub, for if a fungus does gain entry all the growth above this point may be killed. Young Morello cherries, apples and many ornamental shrubs can be lost in this way.

If a large branch tears away, trim the wound by cutting and chiselling to obtain a smooth surface. Should a large cavity be left, it may be necessary to paint its surface with preservative fluid. Fill the wound with a strong ordinary cement mixture and later cover the whole area with a good bituminous paint.

SPACING

Spacing the plants so that each has sufficient room may seem a small point, but hygienically it is important. Thickly sown seedlings are always subject to the familiar ‘damping off’ disease. If they are pricked out too closely together into seed boxes they may still be crippled by-various mildews and moulds. Very close planting of older plants or ‘massing’ in beds may encourage such a disease as smut to attack small bedding dahlias and grey mould to attack godetias, petunias and zinnias.

NEW STOCK

Examine any new plants which have been purchased. Disease is more obvious on active plants than on dormant ones, but with the latter there may be signs that all is not quite normal. Young cabbages may have slightly swollen roots due to attack by club root; young apple trees may already show symptoms of apple canker; young roses may have galls on their roots or canker in their stems; and many bulbs or corms such as tulips, freesias and gladioli may have discolourations on their flesh — inside the outer rough scales — caused by the presence of disease parasites. With a valuable or valued stock of plants, isolate any new arrivals until new growth shows that disease is unlikely to be present. This is especially worth-while with pot and greenhouse plants.

DISINFECTION AND FUMIGATION

A more direct approach to plant health is the disinfection of greenhouses, frames, boxes, pots and even garden tools. Do this as routine without waiting for a disease to be present or suspected.

If a greenhouse crop has suffered from a severe disease, disinfection is essential before a new crop is planted. For example, old tomato plants at the end of the season are heavily infected with leaf mould (Cladosporium fulvum) and some such process as burning flowers of sulphur in the otherwise vacant house is used to kill off the old foliage, the disease fungus and its spores. A clean house is thus obtained and the old stems are removed for burning without spreading live spores.

In other cases empty the house of its plants, then spray with a good disinfectant or with cresylic acid. Use this at a strength of 1 pint in 5 gal. of water plus a wetting agent, such as Agral, and spray it over the interior of the house with the ventilators open. Then shut the ventilators and doors, keep the house closed for three or four days and afterwards leave it open and aired for about two weeks, after which plants can safely be brought back. Always wear goggles and gloves when carrying out this work.

Garden frames can be disinfected with cresylic acid as recommended above, with 2 per cent formalin solution (l pint of 38 per cent commercial formalin in 6 gal. of water) or with some other disinfectant. Water the fluid on the inside of the frames, treating the soil surface as well as the walls and lights. Close the frames for two days, then air for two weeks. The frames should be safe to use when the smell of the chemical has disappeared, but to be quite sure place a small plant in the frame overnight to see if it survives.

Stack pots and boxes and water them with 2 per cent formalin solution or a similar preparation. Then cover them with wet sacks for a day or two. Stakes should be dipped in one of the copper-containing preservative fluids now available, such as Cuprinol; the larger stakes-are best stripped of bark before treatment. Taking such precautions keeps away disease and prolongs the life of the stakes.

Care must be exercised when using a chemical to clean a fruit store, for the smell of the chemical may be picked up by the fruit. It is safer to wash the shelving and boxes in the store with a strong solution of household soda in hot water during the summer.

Fungicides are now also available as ‘smokes’. One such is TCNB (tetra-chloronitrobenzene), which claims to check the activities of the grey mould fungus (Botrytis cinerea) on many green-house plants such as tomatoes, primulas, cyclamen and carnations. The simplicity of operation of smokes is an advantage: just place a small container of the required size in the greenhouse, close the ventilators and ignite the container with a match, then leave and close the door. The fungicide heats and is quickly dispersed as a gas. It condenses in a matter of hours on all parts of the plants (even on the under-surface of the leaves), on the soil and the interior of the greenhouse, so that any fungus parasite present to which the chemical is toxic is either killed or its growth inhibited. In the latter case, repeat the treatment, if necessary.

SOIL STERILIZATION

The soil in greenhouses and even in some beds outdoors can be treated to destroy disease-producing organisms. This process is known as soil sterilization but, in reality, it is only partial sterilization. Soils can be improved in this way, and those which are sick, that is, soils which contain disease organisms, can be rejuvenated. In particular, the continued good health of tomatoes grown in greenhouses depends on this kind of disease control.

Soil can be sterilized by means of chemicals or steam, and full information on this is given in Bulletin No. 22 of the Ministry of Agriculture, Fisheries and Food, London.

Chemical fluids are used to sterilize the soil in small garden greenhouses, usually cresylic acid at 21 per cent or formalin at 2 per cent. Fork the soil over and water the solutions on at 3 or 4 gal. per sq. yd. so as to saturate it. Cover the soil with wet sacks for two days and plant not less than one month later when the vapours have dispersed.

Treat small heaps of soil or compost on a clean floor by watering on one or other of these fluids. Treat each 6-in. layer, cover with wet sacks and leave for two days. Then spread the soil or compost with a clean spade to disperse the vapours, so that it will be ready for use fairly quickly.

Small quantities of sterilized soil can be bought. This is used as a growing medium for plants in small pots and others in the early stages of growth, so that they then have a better chance of escaping disease infection.

In the open air, soil sterilization has a limited application. Where tulips, for example, are planted in the same beds each year there is a danger that there will be a build-up of tulip fire and grey bulb rot diseases. Such beds may sometimes need to be restored by some form of sterilization. If the area is small it is possible to use a chemical such as 2 per cent formalin, but there are now some good soil chemicals available which can be raked into the soil as a powder to give very good results against soil-borne diseases.

SEED TREATMENTS

Always buy seed — and the term in this sense includes bulbs, corms, tubers, etc. which can carry disease in the dormant state — from reliable firms which do their utmost to supply clean, disease-free material. Despite all precautions, though, some diseases which travel on seeds are easily missed, and once infected seeds are sown, the diseases have the opportunity of spreading. With some crops, therefore, precautions are taken by treating the seed before sowing with a proprietary seed dressing. This protective treatment results in the diseases being killed and the seed, bulb, corm or tuber being left uninjured with its capacity for growth unimpaired.

A disease may infect a seed in one of two ways: it may live on the surface of the seed, quite superficially, so that it can be killed easily; or it may penetrate deeply and live in a dormant state inside the tissues (cells) of the embryo plant.

In the first case, the seeds can be disinfected by steeping them in a solution of weak formalin (2 teaspoonfuls to 1 gal. water) or other suitable chemical, or, alternatively, by dusting them all over with an organic mercury compound, with tetrachloronitrobenzene (TCNB) or with tetramethylthiuramdisulphide (TMTD or thiram). These dusts are described by such names as ‘seed saver’ or ‘seed dressing’.

In the second case treatment is more difficult. But good results are obtained against several important diseases by steeping the seed (bulb, corm, etc.) in hot water. The temperature and time of immersion varies with different diseases. This is called the hot water method.

With many diseases, even some of the internal type, good results are obtained by coating the diseased mother corm, bulb, etc. with a fungicidal dust, so that when the corm is planted the parasite is unable to grow out through the chemical into the surrounding soil. Thus it cannot infect the young growth or the young, developing corm. This method is of great value in keeping stocks of gladioli etc. free from disease. It is often possible to sow treated seeds or to plant treated corms in heavily disease-contaminated soil and get good seasonal results, although the disease still remains in the soil.